This invention relates to an imaging machine of the electrostatic or magnetic type (dry or liquid), specifically to a switchable, self-propelled, pulsating magnetic brush which provides greater toner/developer agitation by longitudinal toner agitation, toner recapture, and residual toner development.
Magnetic printing techniques are well known to the copier and facsimile arts. In a typical magnetic printing machine, electrical signals are applied to magnetic recording heads which induce magnetic field variations in the surface of a moving, magnetic recording medium. The field variations produce a latent magnetic image on the surface of the recording medium which is adapted for attracting and retaining magnetic ink particles. An ink toner, which may be in dry particulate form, is applied to the latent magnetic image and may be transferred to paper or other hard copy media. In many respects, magnetic printing is similar to the more common electrostatic, or xerographic, printing wherein toner particles are attracted to the electric fields created by latent charge image on a dielectric medium.
High quality magnetic printing requires that the toner particles be uniformly distributed on the surface of the recording medium. The toner consists of highly mobile, dust-like particles and care must be taken to prevent the spread of these particles to other components of the printing system with resultant degradation of the printed image.
Magnetic brush structures have, for many years, been utilized for the transfer of toner in electrostatic printing machines. The toner, comprising ferromagnetic materials and insulating resins, is attracted to the surface of a hollow, applicator or cylinder (drum) rotatably disposed around a magnetic core. The magnetic core structure rotates with respect to the surrounding cylinder and carries the magnetic particles to the image surface in its magnetic field (development zone).
An imaging machine of the electrostatic type employs a photoconductor which is typically acted upon by six process steps. These steps are charge, expose, toner deposition or development, transfer, residual toner removal or cleaning, and erase.
Early electrostatic imaging machines carried out the development step using a process referred to as cascade development. In cascade development system, the developer mixture is made to fall under gravity to make contact with the latent image on the photo receptor (photoconductor). Cascade development system was capable of developing finer image details but the developed images were uneven. Cascade development eventually gave way to an improvement referred to as magnetic brush development which has proved to be more reliable and capable of superior image quality. Magnetic brush type toner deposition devices serve to continuously attract and transport magnetic imaging material from a supply reservoir to the photoconductor and return unused or spent material to the reservoir.
U.S. Pat. No. 3,040,704 to Bliss, Jun. 26, 1962 disclose the essential elements of current magnetic brush practice. This patent discloses the use of a non-rotating, cylindrical magnetic field producing structure surrounded by a rotatable, non-magnetic sleeve (or drum) which is driven by a suitable driving source, such as a motor of any known type. In the non-rotating magnet type, the poles are stationary and parallel along the axis, which forms a linear brush perpendicular to the direction of the photoconductor movement.
Though historically xerographic image forming apparatus have used stationary magnet and rotating sleeve development devices in the development system where the magnetic brush makes physical contact with the photoconductor, new image-on-image non-contact based image development systems are using the rotating magnet type with fixed or moving sleeve (drum) development systems.
Cleaning devices in imaging machines of the electrostatic type commonly employ mechanical means to remove residual toner from the surface of the photoconductor. Because mechanical means can damage the photoconductor surface, magnetic brush type cleaning is sometimes utilized.
An object of this invention is to provide novel combination of cascaded and magnetic brush structure, for use in magnetic and electrostatic printing systems.
High quality magnetic or electrostatic printing requires that the toner particles be uniformly distributed on the surface of the recording medium. The toner consists of highly mobile, dust-like particles and care must be taken to prevent the spread of these particles to other components of the printing system with resultant degradation of the printed image.
The magnetic brush structures of the prior art, while suitable for the application of toner to electrostatic images, produce a strong magnetic field component in the plane of the recording medium surface. These field components will distort and erase the magnetic latent image in a magnetic printing machine and prevent the use of prior art magnetic brush structures in such magnetic printing machines.
U.S. Pat. No. 3,945,343 to Berkowitz Mar. 23, 1976 discloses an improved magnetic brush for use in magnetic printing which minimizes the interaction between the magnetic brush field and the recorded magnetic image by intelligent placing of pole pieces in a stationary magnet and rotating sleeve (drum) magnetic brush system.
Color capable imaging machines of the electrostatic type develop images using a plurality of magnetic brush type toner deposition devices. Typically, these machines possess one toner deposition device for each of four colors--cyan, magenta, yellow, and black. The machines act to sequentially deposit toner of the first color, followed by toner of the second color followed by toner of the third color, followed by toner of the fourth color. To maize image quality, only one magnetic brush is permitted to deposit toner images on the photoconductor at any given time either by the brush physically touching the photo conductor or by the brush being in close proximity but not touching the photoconductor.
In the traditional contact development process, this sequential imaging process is carried out by physically moving the first color magnetic brush close enough to touch the photo conductor only, followed by the second color magnetic brush only, followed by the third color magnetic brush only, followed by the fourth color magnetic brush only. The four images are transferred on the same paper sequentially in four steps so that the contacting magnetic brush is developing only one color image at any particular time.
In the new non-contact development process, this sequential imaging process is carried out by electrically enabling the first color magnetic brush to form the image on the photoconductor without touching the photo conductor, followed by the second non-contact color magnetic brush only, followed by the third non-contact color magnetic brush only, followed by the fourth non-contact color magnetic brush only. The four images are transferred and overlaid (image on image) first on the same area on the photoconductor in four steps to form the single color image. In this process, the non-contacting magnetic brush or electrostatic brush is used develop one color image on top of the next color. Due to the non-contact nature of the magnetic brush the image on the photo conductor is not smeared. Finally, the color image is transferred to a media or paper in one single step.
An object of this invention is to combine the advantages of cascade and magnetic brush development system in a compact space and to provide very high reliability, high performance and low cost development system. As noted below, few of the prior art inventions disclose methods of combining cascade and magnetic brush development.
U.S. Pat. No. 2,910,963, to Herman, Nov. 3, 1956 discloses a structure using electromagnets to create a pulsating magnetic field over the total area of the photo-sensitive and exposed paper. This pulsating magnetic field produces a constant agitation of developer mix which can be controlled either by varying the intensity of the magnetic field produced or by varying the frequency of the pulsating field in order to obtain optimum results.
U.S. Pat. No. 2,930,351 to Giamo, Mar. 29, 1960, U.S. Pat. No. 3,545,968 to Sato, Dec. 8, 1970, and U.S. Pat. No. 3,117,891 to Lehmann, Jan. 4, 1964, disclose yet another structure for improving the cascade development method for electrostatic images by feeding ferromagnetic carriers in a developing space where the pulsating magnetic field is induced by various methods to improve the image quality, especially solid area coverage in a xerographic machine.
This prior art inventions suffer from the following disadvantages:
(a) they require a great deal of space around the circumference of the photoreceptor; PA1 (b) they are difficult to switch off the development system quickly for color development; PA1 (c) they are not practical for color processes due to the space requirements as noted in (a), as well as (b), above; PA1 (d) the may require external driving means for creating toner agitation; PA1 (e) they are not capable of making dynamic changes to the speed or direction of the developer mixture flow since this prior type of cascade method solely relies on gravity. PA1 (a) to provide uniform magnetic or electrostatic brush at low cost and high reliability; PA1 (b) to provide imaging mixture tumbling properties in the magnetic brush with both cascade and electro-magnetic forces; PA1 (c) to provide means for dynamically varying the speed, direction and intensity of the magnetic field and to selectively and segmentally energize electro-magnets should this prove advantageous for reasons of image quality or efficiency; PA1 (d) to provide (a), (b) and (c) above with sufficient toner or developer mix flow rates so as to be functional in an imaging machine of the electrostatic type; PA1 (e) to provide color development system within less space with fewer parts and actuators and thereby reduce design, manufacturing, and maintenance costs; PA1 (f) to provide smooth (vibrationless) operation with color to color switching and thereby improve image quality, PA1 (g) to eliminate the need to apply an external driving force to the rotating magnet or rotating drum as required in the conventional development system; and PA1 (h) to eliminate the need for a drum as a imaging mixture carrier or deposition structure.